High-quality (Q) filters are the key building block in any receiver (RX) because not only do they provide selectivity also the linearity performance of RX is mainly dependent on the linearity of the filter. A simplified block diagram of the typical front-end 100 of RX is shown in FIG. 1. The wanted RF input voltage at the antenna 107 is usually accompanied by large adjacent channel interferers, out-of-band blockers, and/or transmitted (TX) blockers. The first stage of the front-end is usually a low-noise amplifier (LNA) 101, which provides input matching and gain for the RX band. The amplified RF signal is down-converted to intermediate-frequency (IF) or base-band (BB) by a mixer 103 using a local oscillator (LO) signal 102. The down-converted signal is input to a filter 105 to select wanted signal and filter out blockers and interferers.
There are different types of filters, such as LC, Gm-C, biquad, N-path, and IIR. There are lots of problems associated with these filters. LC filters are very linear, but they do not provide enough selectivity. So they are not applicable for cellular RX. Some other filters such as Gm-C filters not only have a very complex structure but also consume much power compared to filters without active components. Furthermore, input-referred noise of the Gm-C filters is much larger compared to other filters due to the number of active gm cells used. Also, the linearity of the Gm-C structures is worse compared to other structures. Biquad filters are divided into two subcategories, sample-based- and continuous-time filters. In both subcategories, the filter cores are based on operational amplifiers (opamps) or Gm cells. Usually, biquad filters consume much power in active components (i.e., opamps). Also, biquad filters must be made very bulky to reduce flicker noise generated by active devices. An N-path filter concept is based on frequency translation techniques utilizing mixers because the mixers are required to transfer signals in the frequency domain. Therefore, the N-path filter is the low-pass RC filter, which is converted to band-pass by translation via mixer. Also, filters combining the N-path concept with the Gm-C structure have the above mentioned problems. The N-path filters offer a very good selectivity but at the cost of replicas in their transfer functions. These replicas make the filter ineffective to blockers and interferers. Also, input second order intercept (IIP2) of the N-path filters are limited because the generated second order intermodulation product (IM2) in low frequencies coinciding with the wanted signal is up-converted by the mixer.
Charge-sampling infinite impulse response (IIR) filters are based on capacitors and switches only. Therefore, the power consumption in these filters is only related to the power for driving the switches. Also, these filters are less complicated and highly linear because there is no active component (i.e. no operational amplifier) in these filters. However, IIR filters are low-pass, and it is not effective for using these IIR filters in high-IF frequencies.
Hence, it is desired to provide a different kind of filters (for example, Band-Pass-Filter) without the above mentioned problems that would be compatible with the superheterodyne architecture.